1. Field of the Invention
The present invention relates to a data reading method and a data reading apparatus, and, in particular, to a data reading method and a data reading apparatus in which desired read data, stored previously, is obtained in response to issuance of a reading request.
2. Description of the Related Art
For example, a system called PCI-Express has been proposed as an interface system used when data stored in a hard disk drive is read.
PCI-Express was proposed by Intel Corporation, and creation of a specification thereof has been carried on by PCI-SIG. PCI-Express corresponds to a serial interface known by a name ‘3GIO (3rd generation I/O)’.
A PCI bus employed by the most of current computers applies a parallel transfer system. Thereby, no compatibility exists between the current PCI and the above-mentioned PCI-Express on the physical technical level. However, a communication protocol and so forth can be applied in common therebetween.
Further, while a highest communication data rate of the current PCI is 1.06 Gbps, a highest communication data rate of PCI-Express is 2.5 Gbps. Further, since a use in such a manner that a plurality of buses are bound up is allowed, a communication data rate of 5 Gbps can be achieved as a result of two lanes of PCI-Express buses being bound up.
In a protocol of PCI-Express, as will be described later, an order of read data obtained responsive to respective reading requests is not guaranteed. On the other hand, for when data stored in a memory is read, a system may be applied in many cases such that, a time required for obtaining read data from a time of issuance of a corresponding reading request is measured, and, the reading request is issued again when the read data has not been obtained within a predetermined time period, i.e., when time out occurs.
According to the PCI-Express interface, an order of read data responsive to respective reading requests is not guaranteed as mentioned above. Therefore, when a reading request is issued again after time out is detected as mentioned above (referred to as ‘retry’ hereinafter), it may not be possible to determine whether the resultant read data is one responsive to the first reading request or one responsive to the second reading request (retry) issued after the time out detection.
In such a case, since the retry is made once acquisition of read data responsive to the first reading request is given up, a problem may occur when read data is then obtained responsive to the first reading request unexpectedly after the retry, and it is not possible to positively determine whether the read data thus obtained is one responsive to the first reading request or one responsive to the retry. In such a case, proper data may not be obtained, in view of the entire read data obtained responsive to predetermined successive reading requests.
Japanese Laid-open Patent Applications Nos. 2002-41445 and 2005-85079 disclose related arts.